This invention relates generally to the confinement of molten metal and is particularly directed to the vertical casting of metal sheets or rods using an electromagnetic field to form the casting mold.
Steel making occupies a central economic role and represents a significant fraction of the energy consumption of many industrialized nations. The bulk of steel making operations involves the production of steel plate and sheet. Present steel mill practice typically produces thin steel sheets by pouring liquid steel into a mold, whereupon the liquid steel solidifies upon contact with the cold mold surface. The solidified steel leaves the mold either as an ingot or as a continuous slab after it is cooled typically by water circulating within the mold wall during a solidification process. In either case, the solid steel is relatively thick, e.g., 6 inches or greater, and must be subsequently processed to reduce the thickness to the desired value and to improve metallurgical properties. The mold-formed steel is usually characterized by a surface roughened by defects, such as cold folds, liquation, hot tears and the like which result primarily from contact between the mold and the solidifying metallic shell. In addition, the steel ingot or sheet thus cast also frequently exhibits considerable alloy segregation in its surface zone due to the initial cooling of the metal surface from the direct application of a coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or sheet prior to working to remove both the surface defects as well as the alloy deficient zone adjacent to its surface. These additional steps, of course, increase the complexity and expense of steel production.
Steel sheet thickness reduction is accomplished by a rolling mill which is very capital intensive and consumes large amounts of energy. The rolling process therefore contributes substantially to the cost of the steel sheet. In a typical installation, a 10 inch thick steel slab must be manipulated by at least ten rolling machines to reduce its thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Another approach to forming thin metal sheets involves casting into approximately the final desired shape. Compared to current practice, a large reduction in steel sheet total cost and in the energy required for its production could be achieved if the sheets could be cast in near net shape, i.e. in shape and size closely approximating the final desired product. This would reduce the rolling mill operation and would result in a large savings in energy. There are several technologies currently under development which attempt to achieve these advantages by forming the steel sheets in the casting process. While some of the approaches under investigation use electromagnetic energy, all of these approaches use a solid mold on one or both sides of the sheet. One disadvantage of a solid mold is that contact between the molten metal and the solid mold wall often produce an undesirable surface finish which requires subsequent processing to correct as pointed out above.
Previous inventions have employed electromagnetic fields as a substitute for the solid molds. For example, the use of electromagnetic levitation techniques has been employed for some time in the aluminum industry. The practice there is to use electromagnetic fields to contain the top inch or so of a large, thick ingot. The molten aluminum is cooled and solidified before it touches any mechanical support. Examples of this approach can be found in U.S. Pat. Nos. 3,467,166 to Getselev, 4,161,206 to Yarwood et al., and 4,375,234 to Pryor. U.S. Pat. No. 4,678,024 and No. 4,741,383 to Hull et al., were directed toward use of alternating electromagnetic fields to levitate an entire sheet of molten metal for horizontal casting.
There are several difficulties associated with the use of electromagnetic fields as a substitute for solid wall molds. Such difficulties include high energy requirements, large eddy currents, instabilities, and shaping the electromagnetic field to conform to the desired shape of the mold. For example, the Getselev patent describes a device for electromagnetic confinement of a metal, in particular aluminum, as it is cast into rods. The Getselev device employs metallic rings which form screens located at specific positions around the molten metal. These screens serve to shape and modify the magnetic field. The electromagnet of Getselev induces a current in the rings or screens. A frequency is chosen to make the skin depth about 1/3 of the horizontal distance to the center. Eddy currents are generated in the molten aluminum to interact with the applied field and produce a containing force at the surface. In addition to these desirable eddy currents in the aluminum, there are also currents set up in the ring and screen. These currents are responsible for shaping the field but result in large power losses. In addition, the large magnetic fields in the air near the caster may interfere with other equipment and may be a safety hazard.
Another of the previous methods is described in the patents by Hull et al. The Hull et al. patents describe how molten steel could be poured through and solidified in an electromagnetic caster in a horizontal geometry. A horizontal geometry has the advantage of low eddy currents but the stability of the molten metal in the field would be weak.
Accordingly, an object of the present invention is to provide a magnetic field which can retain a molten metal with smooth, even vertical boundary.
It is another object of this invention to provide a casting system for shaping molten metal into various shapes without mechanical contact with a mechanical mold before the metal surface solidifies.
Another object of this invention is to produce steel sheet that requires little or no rolling after the casting operation.
A still further object is to produce steel that has good metallurgical properties and a good surface quality directly upon leaving the caster.
A yet further object of this invention is to provide a casting system with the molten metal in stable mechanical equilibrium within the caster.
A yet still further object of this invention is to provide a casting system for aluminum that uses much less power than existing techniques and confines the magnetic field to the required region.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.